Plastic cover with hinged closure and molding dies therefor

ABSTRACT

The disclosed plastic cover is similar to and an improvement on that shown in U.S. Pat. No. 3,095,995 by virtue of the projection of the peripheral edge of the hinged closure flap laterally and outwardly beyond the full length of an arcuate sealing rib and the opposed wall of the dispensing opening with the outer face of the rib making an acute included angle with the under surface of the flap and the inner rib surface disposed substantially parallel to the path of separation of the die surfaces in which the flap and rib are molded. The molding of such a cover construction is made possible by a unique die assembly which permits the hinged flap and rib to be disposed at a substantial obtuse included angle relative to the dispensing opening during the molding of the cover.

United States Patent Foster [54] PLASTIC COVER WITH IIINGED CLOSURE AND MOLDING DIES THEREFOR [72] Inventor: John A. Foster, Rockford, Ill.

[73] Assignee: J. L. Clark Manufacturing Co., Rockford,

Ill

[22] Filed: May 13, 1970 [21] Appl.N0.: 36,917

[56] References Cited UNITED STATES PATENTS 3,469,732 9/1969 Foster ..220/31S 3,155,285 11/1964 VanBaam 1 July 11,1972

FOREIGN PATENTS OR APPLICATIONS 771,996 1 H1967 Canada ..222/480 Primary Examiner-George E. Lowrance Attorney-Wolfe, Hubbard, Leydig, Voit & Osann Ltd.

[ 5 7] ABSTRACT The disclosed plastic cover is similar to and an improvement on that shown in US. Pat. No. 3,095,995 by virtue of the pro jection of the peripheral edge of the hinged closure flap laterally and outwardly beyond the full length of an arcuate sealing rib and the opposed wall of the dispensing opening with the outer face of the rib making an acute included angle with the under surface of the flap and the inner rib surface disposed substantially parallel to the path of separation of the die surfaces in which the flap and rib are molded. The molding of such a cover construction is-made possible by a unique die assembly which permits the hinged flap and rib to be disposed 10 Claims, 9 Drawing Figures PATENTEDJULH I972 3.675.812

sum 10F 5 m mw/vz. T

minimum 1972 3,675,812

SHEET H [1F 5 m/mwrae. .fmw 4. Fayre-z,-

PLASTIC COVER WITH HINGED CLOSURE AND MOLDING DIES THEREFOR BACKGROUND OF T INVENTION The present invention relates to a plastic cover of the general construction disclosed in my prior U.S. Pat. No. 3,095,995 including a hinged closure flap swingable into and out of a top recess to close or uncover a dispensing opening defined by a straight edge along the hinge and an opposed arcuate wall which receives and telescopes with an arcuate rib depending from the underside of the flap to provide a tight seal around the periphery of the opening when the flap is fully closed.

SUMMARY OF THE INVENTION The present invention improves on the patented construction by making the arcuate sealing rib smaller than the closure flap all around the latter between the ends of the flap hinge so that, in the closed position of the flap, the peripheral edge portion thereof overlies and abuts against the top surface surrounding the opening. A seal of increased effectiveness is formed by the free edge of the rib engaging the opposed and somewhat yieldable wall on the container top.

The foregoing objective is achieved by inclining the outer and inner surfaces of the flap at angles such that in the molding of the cover these surfaces are disposed substantially normal to the path of separation of the molding dies and therefore may be molded with a contour required for achieving the desired sealing action all around the dispensing opening.

Another object is to facilitate such molding and tight sealing by a novel shaping of the ends of the sealing rib and surfaces abutted thereby in the closed position of the flap.

A further object is to provide, in the other end portion of the cover, separate and novelly constructed sifter openings covered by a flap which may be molded in the same inclined position as the spoon hole flap.

The invention also resides in the novel construction of the die assembly for molding of the cover with the spoon hole and sifter flaps inclined at the obtuse angle referred to above while permitting easy removal of the cover after cooling by movement along the axis of the die assembly.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmentary perspective view of a spice can having a cover embodying the novel features of the present invention.

FIG. 2 is a perspective view of the cover with the closure flap in the open position in which it is molded.

FIGS. 3 and 4 are sections taken along the lines 3-3 and 44 ofFIG. 1.

FIG. 5 is a section taken along the line 5-5 of FIG. 2 but with the closure flap in closed position.

' FIG. 6 is a section taken along the line 6-6 of FIG. 2.

FIGS. 7 and 8 are sections taken along the line 7-7 of FIG. 6 with the closure flap in open and closed positions.

FIGS. 9 and 10 are fragmentary sections taken along the lines 9-9 and 10-10 of FIG. 8.

FIG. 11 is a plan view ofFIG. 2.

FIG. 12 is a fragmentary section taken along the line 12-12 of FIG. 1 1.

FIG. 13 is a view similar to FIG. 2 showing a modified form of the cover.

FIG. 14 is a fragmentary section taken along the line 14-14 of FIG. 12.

FIGS. 15 and 16 are perspective views similar to FIGS. 1 and 2 showing another modification.

FIG. 17 is a section taken along the line 17-17 of FIG. 15.

FIGS. 18 and 19 are fragmentary diametrical sections of a die assembly for molding the plastic closure shown in FIGS. 1 to 14.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to the form shown in FIGS. 1 to 12, the invention is incorporated in a container for spices or like materials comprising a single piece generally fiat and rectangular cover 20 of molded resilient plastic such as polyethylene or polypropylene having a depending peripheral flange 21 telescoped over and interlocked as shown in FIGS. 3 to 5 with a hem 22 defining the open end or lip of a tubular sheet metal body 23. In one end portion of the cover is a laterally elongated spoon hole 24 defined by a peripherally continuous skirt 19 depending a short distance from the underside of the cover to provide a flat and straight wall 25, connecting parallel side walls 27, and an arcuate wall 26 defining the front edge of the spoon hole 24. The walls 25 and 26 are spaced inwardly from the long sides of the flange 21 and, together with the latter and depending webs including 17 and 18, impart the desired rigidi ty to the cover while permitting the use of a minimum volume of plastic. The upper edges of walls 26 and 27 together with the flap top area 28 surrounding these walls are disposed somewhat below the level of the top surface 29 of the cover. A shallow recess is thus formed around the dispensing opening 24 for receiving and seating a thin and flat closure flap 30 joined along a thinned section 31 (FIGS. 2 and 4) with the upper edge of the straight wall 25 and forming therewith a hinge for upward swinging of the flap as shown in FIGS. 2 and 6 to uncover the opening 24 or downwardly swinging into the recess and against the bottom 28 thereof.

Between the ends of the hinge, the peripheral edge of the flap may be of any peripheral shape and matches that of the wall of the recess 28 so that the cover fits closely therein and is substantially flush with the top surface 29 when the flap is in closed position (FIG. 1). I-Ierein, the edge portions 32 of the flap extending normal to the ends of the hinge are straight and parallel and merge with straight portions 33 which converge outwardly and terminate at a finger tab 34. Below the outer end of the latter, the top of the cover is recessed deeper as indicated at 35 to provide a thumb hole 36 to facilitate gripping and upward swinging of the flap.

To seal the opening 24 tightly when the flap is closed, a narrow rib 37 of the same curvature as the opening wall 26, 27 but slightly smaller than this wall is molded integral with and depends from the underside of the flap. The rib extends around the full arcuate length of these walls and terminates at the hinge 31 in ends 38. In accordance with one aspect of the present invention, the ends 38 of the adjacent portions 39 of the rib 37 are spaced short distances inwardly and along the hinge from the side edges 32 of the flap 30, and the rib is shaped and dimensioned accurately so that it will fit closely within and around the full lengths of the walls 26, 27. To these ends, the rib is of substantially the same depth as the skirt wall 26, is somewhat thicker and correspondingly less yieldable than the skirt l9, and preferably tapers somewhat toward its free edge 41 as shown in FIGS. 6 and 12. The outer surface 40 of the arcuate portion of the rib 37 is disposed at an acute included angle a and the inner surface 40 is disposed at an obtuse angle c (FIG. 6) relative to the underside of the flap. The magnitude of these angles, for a and for c in the present instance, is such that, by disposing the mold cavity in which the flap is formed at the proper obtuse angle b, about (FIG. 18) relative to the plane of the cover, the rib surfaces 40 and 40 and all of the internal surfaces defining and extending around the hole 24 will be exposed and disposed vertically and thus capable of being molded to provide the improved seal around the spoon hole when the flap is in closed position.

With the rib thus constructed, as above described, it will be apparent that each successive arc of the rib edge 41 will be pressed inwardly a slight amount as it passes and enters the upper edge of the upright hole walls beginning at the ends 27 and progressing around the wall 26 to the center thereof as the flap becomes fully closed. Preferably, the skirt 19 is somewhat thinner than the flap rib and therefore more yieldable than the rib and near its free edge tapers to a relatively thin edge 42. Also, the skirt surfaces 26 and 27 are inclined somewhat inwardly and downwardly as indicated at 26 and 27 (FIGS. 6 and 8) and are disposed at a radii enough shorter than that of the edge 41 of the flap rib 37 to ensure some outward bending of the skirt edge 42 as the flap approaches the closed position.

Because of the resiliency of the plastic and the corresponding shaping and positioning of the skirt 19 and the rib 37 as above described, the lower edge portions thereof will yield laterally as needed in order to allow full telescoping of the two as the flap becomes fully closed. By virtue of the inclining of the surface 26 and the increased yieldability of the lower edge portion of the skirt, the edge 42 of the latter is left compressed tightly against the lower edge of the rib as shown in FIGS. 4, 8 and 12. A tight seal is thus formed all around the flap rib.

To hold the flap securely in closed position, one or more nibs or, in this instance, two arcuate and horizontally disposed beads 45 are formed around the wall 26 of the skirt 19 between the upper and lower edges thereof and project a few thousandths of an inch into the path of the free edge 41 of the rib 37. By yielding of beads and the rib and skirt, the rib edge passes the beads and snaps in beneath the same as shown in FIG. 12 when the flap reaches the fully closed position.

To enable the cover with the flap and rib 37 angularly related as above described to be molded as a single piece and the congealed cover to be removed from the molding as a single piece and the dies, the ends 38 of the rib must be disposed, during the molding, at least at a right angle relative to the plane of the dispensing opening 24. This means that in the closed position of the flap, the ends 38 will be disposed at an angle to the plane of the flap back wall 25 of the dispensing opening 24. Leakage of finely powdered material out of the container and around the ends 38 of the rib when the flap is closed is prevented by forming in the molding operation lugs 46 at opposite ends of the wall 25 with the surfaces 47 inclined downwardly and into the dispensing opening for abutment with the rib ends 38 when the flap is fully closed. Since the inclined surfaces of such lugs face upwardly, they may be formed in the die assembly (FIG. 18) in which the flap including the rib 37 is molded.

To insure more effective sealing at the rib ends 38, shallow and narrow beads 48 are formed along the tops of the inclines 47 for abutment with and compression by the rib ends 38 as the flap reaches its closed position. The effectiveness of the seal is further increased by extending the beads 48 laterally at the lower ends as indicated at 49 in FIGS. 7, 8 and 10. The end 50 of this extension is thus positioned to come into abutment with and be compressed tightly against the opposed wall 27 of the skirt 19 as the flap reaches fully closed position.

Construction of the flap 30 with a sealing rib 37 having parallel end portions 39 as above described is desirable in order to make the opening 24 and the wall 25 thereof wide enough to receive a conventional teaspoon for removing the container contents. For larger size covers such as shown in FIGS. 13 and 14, the wall 26 of the spoon hole may be curved all the way to the straight wall 25, the rib 37 being of arcuate curvature throughout its peripheral length as shown in FIG. 13. As before, the ends 38 of the rib 37'are disposed at no greater than a right angle relative to the dispensing opening in the position in which the flap 30 is molded. As in the construction first described and as the flap reaches closed position, the end portions of the flap rib 37 telescope closely within the wall 26 defining the opening 24, and the rib ends 38 come into tight sealing abutment with shallow beads 48 upstanding from the inclined surfaces of lugs.

In another of its aspects, the present invention contemplates the novel construction of a die assembly which permits the formation in conventional plastic molding procedures of the flap 30 and rib 37 and the rib ends 38 angularly positioned relative to each other as described above. This may be accomplished in a multiple part assembly having a fixed platen 50 and a platen 51 movable forth and back along a path indicated by the arrows between closed and open positions shown in FIGS. 18 and 19. The cavity 52 for shaping the upper and lower surfaces of the flap 30 is disposed during the molding at the obtuse angle b relative to the cavity in which the other parts of the cover including the flange 21 and the skirt 19 are formed. For molding the cover above described, the die assembly comprises four plates 55 to 58 disposed between the platens 50, 51, supported and guided for movements along the cover axis by lost motion connections including annular series of screws 59 and 60 and urged apart by compression springs 62 and 63, which are compressed in the closed position of the die assembly shown in FIG. 18. Opposed surfaces 53 and 54 on the plates 55 and 56 define the cavity 52 in which the flap 30 is molded while opposed surfaces 64, 65 and 66 of the plates 55, 56 and 57 form the cavity for molding the remainder of the cover including the flange 21 which is formed in the plate 57. In accordance with conventional molding practice, molten plastic for forming the cover is injected through a radially extending passage 67 defined by radially extending grooves in the opposed surfaces of the plates 56, 57, the inner end of the passage leading to the cavities through a small hole 68. The plastic congealing in this hole and the passage is broken away from the molded cover and falls away from the same when the die plates are separated, this being in accordance with conventional molding practice.

After injection of molten plastic into and filling the mold cavity, the platen 51 is moved outwardly and away from the fixed platen 50 and to the fully open position shown in FIG. 19. In the initial part of this movement, the plate 56 is separated by the springs 62 from the plate 55 and to the position shown in dotted outline in FIG. 19, the flap 30 being swung inwardly about the edge 69 of the plate 56 and against the side 71 of the plate 55. In this position, the flap is freed for withdrawal through the plate 56 and to the position shown in dot-dash outline as the platen 51 is further retracted. At this time, the flange 21 is still interlocked with the plate 57 after the lost motion provided by the screws 60 has been taken up as shown in dot-dash outline in FIG. 19.

As the retraction of the movable platen 51 continues to the position shown in full at the right in FIG. 19, a plate 72 carrying parallel ejector pins 72 comes into abutment with a stop 73 so that, in the final movement of the platen, the molded and then solidified plastic cover is formed off from the formations on the die plate 57 which shape the cover flange 21. The molded cover is thus moved away from the die plate 57 as shown in full in FIG. 19 and is free to fall downwardly and out of the die assembly. Thereafter, the platen 51 is moved reversely to close the die assembly (FIG. 18) preparatory to another cover forming cycle.

The present invention also contemplates the provision of a series of sifter holes 75 and 76 at the end of the cover opposite the spoon hole 24 and the closure of such holes by projections 77 and 78 formed on the underside of a flap 79 molded in an inclined position (FIG. 2) at the same time as the flap 30 and swingable about an extension of the axis of the latter flap. The flap 79 is flat and rectangular and is swingable down against a surface 81 which is raised to locate the top of the flap when closed (FIG. 1) flush with that of the flap 30.

The holes 75 are arranged in parallel rows along the surface 81 with one intermediate hole 76 centered and located near the outer end of this surface. Frustoconical surfaces 82 at the upper ends of the holes guide the entry of the cylindrical free ends of the projections 77, 78 and provide for seating of complemental surfaces 83 of the projections when the flap is closed, there being small clearance 79 between the flap and the top 81 in the closed position of the flap as shown in FIG. 5. Progressing outwardly along the flap and away from its hinge, the projections 77 78 are made of increasing lengths as shown in FIG. 5, thus allowing for easy entry of all of the projections during closing of the flap and more secure holding of the closed flap by the longer projections. Also, the outer side of the outermost projection 78 is undercut as indicated at 84 so as to interengage with the edge 85 of the hole 76 with a snap overcenter action as the flap reaches closed position.

The flap 79 is somewhat longer than the top surface 81 so that its free end projects beyond this surface thus leaving a recess 86 (FIG. 1) for facilitating gripping of the flap for lifting thereof. This recess is confined to the free end of the flap by forming a rib 87 (FIG. 2) along the outer edge of the flap with the end portion 88 thereof curved around the comer 89 of the surface 81. The space between the flap and the top surface 35 of the cover is thus closed effectually thereby minimizing the danger of the flap being engaged and swung open in the course of handling and shipment.

The sifter flap constructed as above described may be molded at the same time as the flap 30 and in a four part die assembly similar to that shown in FIGS. 18 and 19 and with the cavity for forming the flap, its hinge, and the projections 77, 78 disposed about in the position shown in FIG. 2 and its hinge alined with the hinge of the flap 30. Although the sides of the projections nearest the hinge will face downwardly, the projections will, because of their short length and resiliency, be released from the die part 57 in the separation from the part 56 as shown in FIG. 19.

Closure flaps and sealing thereof as well as the molding technique as above described may be used for other kinds of containers. In the example illustrated in FIGS. -17, the parts corresponding to those first described are indicated by the same but primed reference numerals. In this container, the single piece cover 20 is molded with a depending cylindrical skirt 91 having an internal thread 92 for mating with a thread on the open end of a jar 93 or other container. Screwing of the cover onto the jar is limited by engagement of the jar lip with an annular abutment 94 (FIG. 17) facing downwardly on the underside of the cover.

The top 29 of the cover and the flap 30 are semi-circular and the thinned section 31' defining the flap hinge extends diametrically across the cover between points 95. The surface 26' defining the arcuate portion of the dispensing opening 24' is the inner edge of a flange 96 turned inwardly near the upper end of the skirt as shown in FIG. 17. The skirt 91 is extended upwardly as indicated at 97 beyond the top of this flange so as to provide a recess for enabling another jar to be stacked on top and located laterally. The flange 96 is shaped to provide an upwardly opening groove 97 separating the skirt extension and the outer edge of the flap 30'. Interference with the swinging of the latter is thus avoided.

As in the constructions above described, the entire length of the peripheral edge of the flap 30 projects outwardly beyond the arcuate rib 37' which depends from the underside of the flap and telescopes tightly within the flange surface 26 in the downward swinging of the flap to closed position. Such overhanging of the flap edges and the spacing of the end portions of the rib inwardly from the sides of the flap is made possible as above described by disposing the die cavity in which the flap is molded at the obtuse angle b (FIG. 18) and as shown in FIG. 17 so that the outer surface 40 of the rib is disposed at the acute angle a (FIG. 17) relative to the flap. Because of this inclining, the end surfaces 38' of the rib are correspondingly inclined and disposed relative to the plane of the flap. To provide a seal between these ends and the ends and the upright wall along the straight edge of the opening 24, lugs 46' are formed at the ends of the wall with downwardly and outwardly inclined faces 47' with which the rib ends come into sealing engagement as shown in FIG. 17 when the flap reaches fully closed position. The opening 24' is thus closed effectually.

For use of the cover with some containers, it is desirable to stop the hinge 31 short of the edges of the flap as shown in FIGS. 15 and 16. In such a case, the sealing above described may be achieved by extending the lugs 46' and their inclined surfaces 47 outwardly for abutment, not only with the ends 38' of the rib 37' but also with the ends 98 of the surface 25 which, like the ends 38' are inclined at the obtuse angle b in the molding operation.

When an extremely tight seal around the dispensing opening is not required, a single hold-down bead 99 may be employed. In this instance, this bead is of short arcuate length and projects outward from the central part of the flap rib 37' as shown in FIG. 16. As the flap reaches closed position, this bead is compressed, as permitted by yielding of the rib and the flange surface 26', and allowed to pass across and snap in beneath this surface as shown in FIG. 17.

For molding the cover shown in FIGS. 15-17, only the three plates 55, 56, and 58 above described are required, the assembly being modified however to provide for unscrewing of the cover from the plate 58 or, in some other conventional way, effecting removal of the molding from the die plate in which the thread 92 is formed.

I claim: 7

1. A single piece container closure of molded resilient plastic having, in combination, a generally flat top surface defining a dispensing opening of generally segmental shape and having a straight edge along one side, the opposite side thereof all around the opening between the ends of said hinge,

a radially flexible rib of narrow width and of the same peripheral contour and length as said wall depending from the underside of said flap around the full periphery thereof between the ends of said hinge and adapted to telescope with a tight continuous interference fit and seal around the full periphery of said curved wall as the flap is swung to said closed position, such tight fitting and sealing of said rib against said wall being achieved by virtue of the outer peripheral surface of said rib, near the free edge and over substantially the full depth of the rib, making an acute included angle with the surrounding underside of the flap so that said rib, beginning at the free edge thereof, is bent inwardly during entry into said opening and telescoping within said wall.

2. A molded container cover as defined in claim 1 in which said acute included angle is of such magnitude as to permit the mold parts, by which the surfaces defining such angle are formed, to be separated by relative bodily movement along a straight line perpendicular to the plane of said opening.

3. A molded container cover as defined in claim 1 in which said acute included angle is such that the full peripheral length of the curved outer surface of said flap rib is vertically disposed when said flap is disposed at a predetermined obtuse molding angle relative to the plane of said opening whereby to permit, without substantial interference, vertical separation of the die surfaces by which the surface of said opening wall and the surfaces of said rib are formed.

4. A molded container cover as defined in claim 1 in which the magnitude of said acute angle is approximately 5. A molded cover as defined in claim 1 in which the elements of the inner surface of said rib are substantially vertical when said flap is open to its molding position.

6. A molded container cover as defined in claim 1 in which the free edge of said rib interlocks with the free edge portion of said curved wall as the cover is swung to closed position thus holding the cover releasably against said top surface.

7. A molded container cover as defined in claim 6 in which the curved wall of said dispensing opening is disposed substantially normal to the plane of the opening and is formed with at least one shallow and inwardly projecting bead disposed intermediate the upper and lower edges of said curved wall of said dispensing opening and acting as the flap approaches closed position to compress the passing rib and snap in beneath the rib and hold the flap in said closed position against said top surface.

8. A single piece container closure of molded resilient plastic having, in combination, a generally flat top defining a dispensing opening of generally segmental shape and having a straight edge along one side, the opposite side edge of the opening being defined by a curved upright wall merging at its ends with the outer ends of said straight edge, a thin flap molded integrally with said straight edge so as to form a hinge along said straight edge permitting upward swinging of the flap from a closed position covering said opening and in which the peripheral edge portion of the flap abuts against said top surface to close said opening and overlaps the edge thereof all around the opening between the ends of said hinge, a radially flexible rib of the same peripheral contour and length as said wall depending from the underside of said flap around the full periphery thereof between the ends of said hinge and adapted to telescope with tight continuous interference around the full periphery of said curved wall as the flap is swung to said closed position, said rib being of relatively narrow radial width and coacting with said curved wall to telescope within said opening with a tight fit, and lugs formed at the ends of said curved wall and having surfaces inclined downwardly from the plane of the cover top and away from said hinge, the ends of said flap rib being shaped to abut said lug surfaces as said flap reaches closed position.

9. A molded container cover as defined in claim 8 including a bead of narrow width molded on one of the opposed inclined surfaces of said lugs and rib ends and adapted to yield more readily as said flap becomes closed whereby to increase the effectiveness of the seal at the ends of the flap rib.

10. A molded container cover as defined in claim 9 including a lateral and outward extension of said sealing bead at the lower portion thereof, such extension projecting into tight.

abutment at its end with the opposed surface of the wall of said dispensing opening at the junction thereof with said hinge. 

1. A single piece container closure of molded resilient plastic having, in combination, a generally flat top surface defining a dispensing opening of generally segmental shape and having a straight edge along one side, the opposite side edge of the opening being defined by a curved upright wall merging at its ends with the outer ends of said straight edge, a thin flap molded integrally with said straight edge so as to form a hinge along said straight edge permitting upward swinging of the flap from a closed position covering said opening and in which the peripheral edge portion of the flap abuts against said top surface to close said opening and overlaps the edge thereof all around the opening between the ends of said hinge, a radially flexible rib of narrow width and of the same peripheral contour and length as said wall depending From the underside of said flap around the full periphery thereof between the ends of said hinge and adapted to telescope with a tight continuous interference fit and seal around the full periphery of said curved wall as the flap is swung to said closed position, such tight fitting and sealing of said rib against said wall being achieved by virtue of the outer peripheral surface of said rib, near the free edge and over substantially the full depth of the rib, making an acute included angle with the surrounding underside of the flap so that said rib, beginning at the free edge thereof, is bent inwardly during entry into said opening and telescoping within said wall.
 2. A molded container cover as defined in claim 1 in which said acute included angle is of such magnitude as to permit the mold parts, by which the surfaces defining such angle are formed, to be separated by relative bodily movement along a straight line perpendicular to the plane of said opening.
 3. A molded container cover as defined in claim 1 in which said acute included angle is such that the full peripheral length of the curved outer surface of said flap rib is vertically disposed when said flap is disposed at a predetermined obtuse molding angle relative to the plane of said opening whereby to permit, without substantial interference, vertical separation of the die surfaces by which the surface of said opening wall and the surfaces of said rib are formed.
 4. A molded container cover as defined in claim 1 in which the magnitude of said acute angle is approximately 80*.
 5. A molded cover as defined in claim 1 in which the elements of the inner surface of said rib are substantially vertical when said flap is open to its molding position.
 6. A molded container cover as defined in claim 1 in which the free edge of said rib interlocks with the free edge portion of said curved wall as the cover is swung to closed position thus holding the cover releasably against said top surface.
 7. A molded container cover as defined in claim 6 in which the curved wall of said dispensing opening is disposed substantially normal to the plane of the opening and is formed with at least one shallow and inwardly projecting bead disposed intermediate the upper and lower edges of said curved wall of said dispensing opening and acting as the flap approaches closed position to compress the passing rib and snap in beneath the rib and hold the flap in said closed position against said top surface.
 8. A single piece container closure of molded resilient plastic having, in combination, a generally flat top defining a dispensing opening of generally segmental shape and having a straight edge along one side, the opposite side edge of the opening being defined by a curved upright wall merging at its ends with the outer ends of said straight edge, a thin flap molded integrally with said straight edge so as to form a hinge along said straight edge permitting upward swinging of the flap from a closed position covering said opening and in which the peripheral edge portion of the flap abuts against said top surface to close said opening and overlaps the edge thereof all around the opening between the ends of said hinge, a radially flexible rib of the same peripheral contour and length as said wall depending from the underside of said flap around the full periphery thereof between the ends of said hinge and adapted to telescope with tight continuous interference around the full periphery of said curved wall as the flap is swung to said closed position, said rib being of relatively narrow radial width and coacting with said curved wall to telescope within said opening with a tight fit, and lugs formed at the ends of said curved wall and having surfaces inclined downwardly from the plane of the cover top and away from said hinge, the ends of said flap rib being shaped to abut said lug surfaces as said flap reaches closed position.
 9. A molded container cover as defined in claim 8 including a bead of narrow width molded on one of the opposed inClined surfaces of said lugs and rib ends and adapted to yield more readily as said flap becomes closed whereby to increase the effectiveness of the seal at the ends of the flap rib.
 10. A molded container cover as defined in claim 9 including a lateral and outward extension of said sealing bead at the lower portion thereof, such extension projecting into tight abutment at its end with the opposed surface of the wall of said dispensing opening at the junction thereof with said hinge. 